Urine formation occurs in the kidneys, or rather in the minimal structural unit of the kidney - the nephron. The nephron consists of a glomerulus and a renal tubule. The glomerulus is formed by a bundle of capillaries, which are branches of the inflowing and outflowing arterioles. The capillaries are surrounded by Bowman's capsule formed by the tubular epithelium. From it, the convoluted sections of the renal tubules begin, passing into the straight tubules.

urination occurs in two phases.

The first phase is filtration. It flows in a capsule and consists in the formation of primary urine. It is assumed that primary urine is filtered from the capillaries of the Malpighian glomerulus into the capsule cavity.

In the second phase of urine formation - reabsorption - in the tubules of the nephrons, there is a reverse absorption (reabsorption) from the primary urine into the blood of amino acids, glucose, vitamins, most of the water and salts.

Glomerular filtration - this is the first stage of urination, which consists in the transfer of fluid and substances dissolved in it from the glomerular capillaries into the capsule cavity

Filtration pressure is the effective pressure, i.e. it is the difference in hydrostatic pressure in the capillaries, which promotes filtration and, preventing filtration, oncotic blood pressure and the hydrostatic pressure of the primary urine itself in the glomerulus of the kidney.

The filtrate entering the Shumlyansky-Bowman capsule is primary urine, which in its content differs from the plasma composition only in the absence of proteins. primary urine, containing water necessary for the body and substances dissolved in it, most of which are of biological value, for example, amino acids, carbohydrates, salts, etc.

Tubular reabsorption and secretion. Threshold substances. Final urine composition. Diuresis.

Tubular secretionis called the active transport into the urine of substances contained in the blood or formed in the cells of the tubular epithelium, for example, ammonia.

Tubular reabsorption - the ability of renal tubular cells to reabsorb substances from the lumen of the nephron into the blood.

All substances contained in blood plasma can be divided into threshold and nonthreshold. TO threshold substances include those that are released in the final urine only when they reach a certain concentration in the blood; for example, glucose enters the final urine only if its blood level exceeds 6.9 mmol / L.

Urine is usually clear, but has a small precipitate obtained by centrifugation and consisting of a small number of erythrocytes, leukocytes and epithelial cells. Protein and glucose in the final urine are practically absent. In small quantities, derivatives of the products of putrefaction of proteins in the intestine - indole, skatole, phenol, enter the urine. Urine contains a wide range of organic acids, small concentrations of vitamins (except for fat-soluble ones), biogenic amines and their metabolites, steroid hormones and their metabolites, enzymes and pigments that determine urine color.

Diuresis - the volume of urine generated over a certain period of time.

A vital process in the kidneys is the formation of urine. It includes several components - filtration, absorption, excretion. In the event of a violation for some reason of the mechanism of production and subsequent excretion of urine, various severe ailments appear.

The composition of urine includes water and special electrolytes, in addition, end products of metabolism in cells are an important component. The products of the last stage of metabolism enter the bloodstream from the cells at a time when it circulates through the body and is excreted by the kidneys as part of urine. The mechanism of urine production in the kidneys is realized by the functional unit of the kidney - the nephron.

The nephron is a unit of the kidney that ensures the formation of urine and its further excretion, due to its versatility. Each organ contains about 1 million such units.

The nephron, in turn, is divided into:

• glomerulus
• bowman-Shumlyansky capsule
• tubule system

The glomerulus is a whole network of capillaries that are embedded in the Bowman-Shumlyansky capsule. The capsule is formed of double walls and resembles a cavity with a continuation into the tubules. The tubules of the renal unit form a kind of loop, the parts of which perform the necessary inherent functions for the formation of urine. The portions of the tubules, convoluted and straight, adjacent directly to the capsule, are called proximal tubules. In addition to these basic structural units of the nephron, there are also:

• rising and falling thin sections
• distant rectus tubule
• fat bearing segment
• henle loops
• distant convoluted tubule
• connecting tubule
• collecting tube

Primary urine production

The blood that enters the glomeruli of the nephron, under the influence of diffusion and osmosis, is filtered through the specific membrane of the glomeruli and in this process spends most of the fluid. The filtered blood products are then fed to the Bowman-Shumlyansky capsule.

All kinds of toxins, glucose, salts, water and various other biochemical substances filtered from the blood and located in Bowman's capsule are called primary urine. contains a large amount of glucose, creatinine, amino acids, water and other low molecular weight compounds. An excellent tubule is 130 ml per minute. If you make simple calculations, it turns out that the nephrons, which are part of the kidneys, filter about 185 liters in 24 hours.

This is a huge amount, because there is not a single case of excretion of such a large amount of liquid. What else is hidden in the mechanism of urine formation?

Secondary urine and its formation

Reabsorption is the second component of the mechanism that causes the formation of urine. This process consists in the movement of various filtered substances back into the capillaries and blood vessels of the circulatory system. The reabsorption process begins in the tubules adjacent to Bowman's capsule, and has its continuation already in the loops of Henle, as well as in the distant convoluted tubules and the collecting duct.

The mechanism of formation of secondary urine is quite complex and painstaking, however, about 183 liters of fluid per day from the tubules back into the bloodstream.

All valuable and nutrients do not disappear along with urine, they all undergo a reabsorption mechanism.

Glucose must return to the blood, provided that there are no disturbances in the body's systems. If the glucose content in the bloodstream exceeds 10 mmol / l, then glucose begins to be excreted along with the urine.

In addition, various ions are returned, including sodium ions. The amount that the kidney resorbs per day directly depends on how much salty the patient ate the day before. The more sodium ions enter the body with food, the more is absorbed from the primary urine.

In a healthy state of the body, urine should not contain protein, erythrocytes, ketone bodies, glucose, bilirubin. If various substances are contained in the excreted urine, this may indicate a malfunction in the liver, gastrointestinal tract, pancreas and many others.

The process of excreting urine from the body

The third important process is tubular secretion. This is the mechanism of urine formation. During this process, hydrogen, potassium, ammonia ions, as well as some drugs, are released from the capillaries near the distant and collecting tubules, into the depression of the tubules, namely into the primary urine, by the method of active transfer and penetration. As a result of absorption and excretion in the renal tubules of primary urine, secondary urine is formed, which should normally be from 1.3 to 2.3 liters.

Excretion in the kidney tubules plays a very important role in stabilizing the acid-base balance of the human body.

The accumulated urine in the bladder increases the pressure in the bladder itself. It is innervated by the autonomic nervous system and, in turn, irritation of the parasympathetic pelvic nerves leads to a contraction of the bladder walls and subsequent relaxation of the sphincter, which entails the expulsion of urine from the bladder.

Urine production largely depends on the level of blood pressure, blood filling of the kidneys, as well as the size of the lumen of the arteries and veins of the kidneys. A drop in blood pressure, as well as a narrowing of the lumen of the capillaries in the kidneys, entails a significant reduction in urine output, and the expansion of the capillaries and, accordingly, increased blood pressure - increase.

It would seem a childish question: why does a person need urine? But everything is much more complicated. In addition to getting rid of dangerous and harmful metabolic products, urination is necessary to maintain electrolyte balance, control the amount of fluid in the body and regulate pressure, as well as the work of the heart and blood vessels. To understand how all this happens, you need to understand a little about the formation of urine.

The formation of primary urine begins with the entry of blood into the kidneys and its movement through the vessels. The kidney at this time plays the role of a filter that passes through the pores all substances that have entered the kidney. Most of the processes of formation of primary urine occurs in the malpighian glomeruli of the kidney. Blood is delivered to the kidneys through the renal arteries. In 24 hours, all the blood in the kidneys is filtered about 20 times.

It is important to understand that the fibrous capsule of the kidney consists of three layers:

1. In the first layer, consisting of capillaries, there are large pores through which all blood passes, with the exception of some proteins and shaped particles.
2. In the second layer consists of filaments of collagen and is a membrane that does not allow proteins to pass through.
3. Finally, in the third layer is epithelial, its cells have a negative charge and do not allow blood albumin to pass into the primary urine. All filtered blood enters the kidney tubules. This is the primary urine.

Due to this, there are no proteins in the resulting primary urine, and the kidneys filter and restore negative elements, leading them to a normal state. Thus, primary urine is a protein-free filtrate of blood plasma. Thanks to all these processes, pressure is also formed in the body.

The normal state of filtration of the primary composition per day is almost one and a half thousand liters of blood (more precisely, 1400). This is followed by the formation of a primary liquid (it is obtained up to 180 l). But no one excretes such an amount of urine in 24 hours.

Reabsorption

This is the formation of secondary urine. Now all the elements move into the blood from the tubules. All proteins that have entered the filtrate, as well as other particles and components in the ultrafiltrate, are subject to reabsorption; it occurs through dysfusion or active transportation.

As a result of active transport, a very high oxygen consumption occurs. During reabsorption, substances and elements from the kidney canals are returned to the blood. Thus, almost all of the primary urine returns to the bloodstream. 160 liters are converted into 1.5 liters of a concentrate called secondary urine. The composition of the secondary urine includes:

• ammonium salts;
• urea;
• creatinine;
• acids;
• other salts.

The result of this whole process is the entry of secondary fluid into the bladder. It gets here through the ureters.

Comparison of secondary and primary urine

№	Signs	Primary urine	Secondary urine
1.	How many liters are formed	Formed in quantities up to 200 liters in 24 hours.	Up to two liters per day.
2.	Where is formed	In the malpighian glomeruli of the kidney	In the tubules of the nephrons
3.	Glucose content	Contained	Not contained
4.	Components of blood plasma (percentage)	As much as in blood plasma, excluding fat and protein	More than plasma. Proteins and fats are also missing.
5.	Does it stand out in the external environment	It is not released into the external environment	Does not stand out

Secretion

The third and no less important stage in the formation of urine. This process is similar to reabsorption going in the opposite direction. The secretion process is quite active, in parallel with it, reabsorption occurs. Secretion is carried out in the renal tubules and in the capillaries of the kidneys. With the help of the distal and collecting canals, ammonia, salts and hydrogen (all in ions) are secreted into the urine. Thanks to this process, unnecessary substances are released from the body through the urethra, which are partially absorbed into the blood. Daily urine dose. The secretion released due to the secretion can be from a liter to two.

Features of the formation of urine in children

In the smallest, by the time of birth, many functional and structural changes in the kidneys are not yet complete, which affects the formation of urine. Here are some of the main features:

• The body weight of a child is greater than that of an adult: for example, the kidney weighs 1 percent of the total body weight. But there are as many nephrons as in an adult, but they are much smaller. As for the epithelial layer on the basement membrane of the glomerulus, it is a tall cylindrical cell. Their filtration surface is reduced, and the resistance is strong.
• In infants, the epithelium of the kidneys is not completely prepared for secretion, and the tubules are short and narrow. The renal apparatus (its morphological structure) matures in babies only by the age of three, and sometimes much later. So, the urine of a small child from an adult differs both in composition and quantity.
• During the first months of a child's life, smaller volumes of fluid are filtered in his kidneys, but urine (if calculated per kilogram of body weight) is formed in larger volumes than in adults. At the same time, the kidneys are not yet able to free the body from an excess of fluid.
• A one-year-old child secretes 0.75 liters of urine per day, a five-year-old - about one liter, a ten-year-old almost as much as adults. Reabsorption processes in children are not as smooth and perfect as in adults: in order to remove toxins, the child needs much more fluid. The child's secretion is also poorly developed. Since the tubules have not yet formed, they do not quite cope with the conversion into the acidic phosphate salt from the primary urine.
• Inferior to adults and the synthesis of ammonia, as well as the reabsorption of bicarbonates and the release of acid residues, which can lead to acidosis. In addition, children usually have a low urine specific gravity.

The formation of urine is a complex and intense process. It involves all parts of the kidney, as well as the ureters, aorta and arteries. As a result, the body gets rid of unnecessary substances, and pressure is also formed. Finally, all its mechanisms are formed only at the age of six.

The mechanism of urine formation is the vital process carried out by the kidneys includes three components: filtration, reabsorption and secretion... Violations in the implementation of the mechanism of formation and excretion of urine are manifested in the form of serious diseases.

Urine consists of water, certain electrolytes and metabolic end products in cells. The end products of metabolism from cells enter the blood during its circulation throughout the body and are excreted by the kidneys as part of urine. The mechanism of urine formation in the kidneys is realized by the nephron.

Nephron - morphofunctional unit of the kidney, which provides the mechanism of urination and excretion. Each kidney contains over 1 million nephrons. In the structure of the nephron, the following parts are distinguished: the glomerulus, Bowman's capsule, the tubular system. The glomerulus is a network of arterial capillaries immersed in Bowman's capsule. Double walls of the capsule form a cavity, the continuation of which are the tubules. The tubules of the nephron form a loop, the individual parts of which perform certain functions in the mechanism of urine formation. The convoluted and straight part of the tubules adjacent to Bowman's capsule is called the proximal tubule. This is followed by a descending thin segment, an ascending thin segment, a distal straight tubule or thick ascending segment of Henle's loop, a distal convoluted tubule, a connecting tubule, and a collecting tube.

The mechanism of urine formation begins with the process
filtration in the renal glomeruli
and the formation of primary urine.

The essence of the filtration process is as follows:
The blood entering the glomeruli, under the action of osmosis and diffusion, is filtered through the specific membrane of the glomeruli and loses most of the liquid and soluble both useful chemicals and toxins. The product of filtration of blood in the glomeruli enters Bowman's capsule. Water, slags, salt, glucose, and other chemicals that are filtered from the blood into Bowman's capsule are called primary urine... Thus, primary urine consists of water, excess salts, glucose, urea, creatinine, amino acids, and other low molecular weight compounds. Normally, the total glomerular filtration rate (GFR, for all nephrons of both kidneys) is about 125 ml per minute. This means that about 125 ml of water and solutes enter Bowman's capsule and the tubular apparatus of the kidney from the blood per minute. For an hour of realization of the mechanism of formation of primary urine, the kidneys filter 125 ml / min x 60 min / hour \u003d 7500 ml, per day, respectively, 7500 ml / h x 24 h / day \u003d 180,000 ml / day or 180 l / day!

Obviously, no one ever excretes 180 liters of urine per day. Why? Because the mechanism of urine formation includes the process of tubular reabsorption, in the implementation of which almost all of this volume of primary urine is returned to the blood.

Renal tubular reabsorption.
The mechanism of formation of primary urine.

Reabsorption is the second component of the urine formation mechanism, by definition, is the movement of substances from the renal tubules back into the blood capillaries surrounding the tubules (called peritubular capillaries). In the mechanism of the formation of primary urine, the properties of the structures of the epithelial cells of the tubules are realized to absorb water, glucose and other nutrients, sodium (Na +) and other ions and secrete them into the blood. Reabsorption begins in the proximal tubules and continues in the loop of Henle, distal convoluted tubules, and collecting ducts.

With the implementation of the complex mechanism of the formation of secondary urine, more than 178 liters of water per day from the proximal tubules returns to the blood.

None of the valuable nutrients are lost in the urine; they are all reabsorbed, including glucose. All is normal glucose (blood sugar) is completely returned to the blood. In the event that the blood glucose exceeds 10 mmol / L (baked threshold), then that part of the glucose is excreted in the urine. Sodium ions (Na +) and other ions are partially returned to the blood. So, the amount of reabsorbed sodium ion largely depends on how much salt is consumed. The more salt comes from food, the less sodium is reabsorbed from the primary urine. The less salt, the more sodium is absorbed back into the blood and the amount of salt in the urine decreases.

Secretion in the renal tubules
as the third component
mechanism of urine formation

Third important process in the mechanism of urine formation - tubular secretion. Tubular secretion is a process in which from the capillaries around the distal and collecting tubules, into the tubular cavity, i.e. in the primary urine, by active transport and diffusion, hydrogen ions (H +), potassium ions (K +), ammonia (NH 3) and some drugs are secreted. As a result of the processes of reabsorption and secretion in the renal tubules of primary urine, secondary urine is formed. The daily volume of secondary urine is normally 1.5 - 2.0 liters.

Tubular secretion in the kidneys plays an important role in maintaining the acid-base balance of the body. Thus, the formation of urine is carried out by the sequential implementation of the processes of filtration, reabsorption and secretion in the nephrons of the kidney.

Their functions include removing unnecessary metabolic products and foreign substances from the body, regulation of the chemical composition of body fluids by removing substances, the amount of which exceeds current needs, regulation of water content in body fluids (and thus their volume) and regulation of the pH of body fluids .

The kidneys are abundantly supplied with blood and homeostatically regulate blood composition ... Thanks to this, the optimal composition is maintained tissue fluid , and, consequently, the intracellular fluid of the cells washed by it, which ensures their efficient work.

The kidneys adapt their activity to changes in the body. Moreover, only in the last two departments nephron - in distal convoluted tubule of the kidney and collecting tube of the kidney - functional activity changes in order to regulation of the composition of body fluids ... The rest of the nephron up to the distal tubule functions in the same way in all physiological conditions.

The end product of kidney activity is urine , the volume and composition of which varies depending on the physiological state of the organism.

Each kidney contains about one million structural and functional units (nephrons). A schematic of the nephron is shown in Fig. No. 1

Figure № 1. The structure of the renal glomerulus and nephron with blood vessels:

1-bringing artery; 2-outflow artery; 3-glomerular capillary network; 4-capsule Bowman; 5-proximal tubule; 6-distal tubule; 7. collecting ducts; 8-capillary network of the cortex and medulla of the kidneys.

The blood plasma that has entered the kidneys (about 20% of the total cardiac output) is ultrafiltered in the glomeruli. Each glomerulus contains renal capillaries surrounded by Bowman's capsule. The driving force providing ultrafiltration is a gradient between blood pressure and hydrostatic pressure of the glomerular space, which is about 8 kPa. Ultrafiltration is counteracted by an oncotic pressure of about 3.3 kPa created by dissolved plasma proteins, which themselves practically do not undergo ultrafiltration (Fig. No. 2).

Figure № 2. Forces providing filtration of plasma in the glomeruli of the kidneys

	Figure № 3. Urinary organs
	kidney cortex medulla
	renal cups
	pelvis
	ureter
	bladder
	Urethra

The process of urine formation takes place in two stages. The first one takes place in the capsules of the outer layer of the kidneys (renal glomerulus). All the liquid part of the blood that enters the glomeruli of the kidneys is filtered and enters the capsules. This is how primary urine is formed, which is practically blood plasma.

Along with the products of dissimilation, the primary urine contains both amino acids and glucose, and many other compounds necessary for the body. Only proteins from blood plasma are absent in primary urine. This is understandable: after all, proteins are not filtered.

The second stage of urine formation is that the primary urine passes through a complex system of tubules, where the substances and water necessary for the body are sequentially absorbed. Everything harmful to the life of the body remains in the tubules and in the form of urine is excreted from the kidneys through the ureters into the bladder. This final urine is called secondary urine.

How does this process take place?

Primary urine passes continuously through the convoluted renal tubules. The epithelial cells that make up their walls do a tremendous job. They actively suck up a large amount of water and all the substances the body needs from the primary urine. From the epithelial cells, they return to the blood flowing through the capillary network that wraps around the renal tubules.

How great is the work done by the renal epithelium can be judged, for example, by the fact that its cells absorb about 96% of the water contained in the primary urine. The cells of the renal epithelium spend a huge amount of energy on their work. Therefore, the metabolism occurs in them very intensively. This is confirmed by the fact that the kidneys, which make up only 1/160 of our body weight, consume approximately 1/11 of the oxygen entering it. The resulting urine flows through the tubes of the pyramids to the papillae and seeps through the holes in them into the renal pelvis. From there, it flows down the ureters into the bladder and is removed outside (Fig. No. 3).